1-Voltage, 2-Wire Secondary

[LarryFine]

Maybe not immediately, but I will. I'll message when/if I hear from anyone.

No! Right now!




j/k

 

[kwired]

Basically this is telling me that I can't wire this particular transformer for 120V/240V output and supply only 120V conductors from that. That the transformer secondary has to be wired for a 120V 2-wire output and supply 120V two-wire secondary conductors... which you would think would be obvious, but alas.

120/240 output supplying 120 only is very common. You must have separate secondary protection in that situation.

 

[kwired]

Although I still think "two-wire, single voltage" may be about "as manufactured."

If you think about it... if for whatever reason, one of the two coils on the secondary side failed, you would be passing the full load through only half of the secondary and this wouldn't trip the primary before the transformer incurred irreparable damage (as also pointed out by other previous posts by others that I don't feel like citing but will give credit to).

Chances are transformer is toast anyway unless you only want to use the half that is still working, and also good chance the second half fails before the problem is noticed.

I never meant to convey modification in saying "field wired." I'm in agreement in that it is being wired the way it was manufactured to be... I'm just not 100% solid on the idea that this "field wiring" (even as intended by the manufacturer) equals "2-wire (single voltage) secondary."

Part of me still feels like... and this is why I brought up the potential failure above... it would be better to have a single coil 120V secondary rated for 3kVA as opposed to a two coil 120V secondary wired in parallel rated for 3kVA.

That logic doesn't correspond to what is most commonly available, so there must not be too many problems with the "dual voltage" arrangement that is pretty common.

Yes, but in the case of a 20A OCPD on the secondary, with two 120V coils wired in parallel, and rated for 3kVA cumulatively, one failing could result in the other pulling up to 2400VA on a 1500VA rated coil without tripping the secondary OCPD. With a single coil rated for 3kVA, it could never surpass the rated kVA without tripping the secondary OCPD.

Unless I misunderstand transformer kVA ratings, which is entirely possible, and both secondary coils are rated for 3kVA in a single transformer rated for 3kVA.

The 3 kVA rating is total rating of all segments, whether talking about primary or secondary. If you intentionally only use half of primary or half of secondary for whatever reason then you only have half of nameplate rating available or you overload it.

 

[jim dungar]

Unless I misunderstand transformer kVA ratings, which is entirely possible, and both secondary coils are rated for 3kVA in a single transformer rated for 3kVA.

Effectively each winding is at the rated for the maximum kVA at the maximum voltage. However they are really rated for current,
In your OP, the primary windings can be connected as series or parallel, but in either case each winding sees only 6.25A at the 240V across it. Likewise each secondary winding only sees 12.5A at 120V.

 

[Jerramundi]

120/240 output supplying 120 only is very common. You must have separate secondary protection in that situation.

That's exactly what I'm saying.

This section of code is telling me that, if I want to NOT have secondary conductor protection, two conditions must be met regarding (1) the transformer secondary conductors and (2) the transformer secondary itself.

The moment I wire this transformer for a single phase 3-wire output, secondary conductor protection is required.

The part that's being debated is the differences between sentence #1 and sentence #2 of 240.4(F). I'm saying sentence number #1 gives conditions for the secondary conductors and sentence #2 gives conditions for the actual secondary of the transformer... both of which are conditions for either requiring secondary conductor protection or allowing secondary conductors to go without.

 

[Jerramundi]

120/240 output supplying 120 only is very common. You must have separate secondary protection in that situation.

Essentially, I read 240.4(F) like this....

1) Single Phase (2-wire) secondary conductors...

A) ...supplied by the secondary side of a single phase transformer having a 2-wire (single voltage) secondary...​

​

B) ...supplied by the secondary side of a three phase, delta-delta connected transformer having a 3-wire (single voltage) secondary...​

​

...shall be permitted to be protected by the primary OCPD.​

​

2) Multiphase (delta-delta, 3-wire) secondary conductors....

A) ...supplied by the secondary side of a single phase transformer having a 2-wire (single voltage) secondary...​

​

B) ...supplied by the secondary side of a three phase, delta-delta connected transformer having a 3-wire (single voltage) secondary...​

​

...shall be permitted to be protected by the primary OCPD.​

​

*Catch-all* ...provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

 

[Jerramundi]

That logic doesn't correspond to what is most commonly available, so there must not be too many problems with the "dual voltage" arrangement that is pretty common.

Or, going without secondary conductor protection is supposed to be limited to only a few special circumstances, which is exactly what 240.4(F) is telling us.

 

[Jerramundi]

Effectively each winding is at the rated for the maximum kVA at the maximum voltage. However they are really rated for current,
In your OP, the primary windings can be connected as series or parallel, but in either case each winding sees only 6.25A at the 240V across it. Likewise each secondary winding only sees 12.5A at 120V.

When you say "each winding is at the rated for the maximum kVA at the maximum voltage," do you mean each INDIVIDUAL COIL (1 out of 4)...

OR "each winding" as in "either the primary or secondary side(s)" because there is no way to wire this particular xfmr to manufacturer's specifications without utilizing both coils on either side (i.e. wired in series or parallel to produce the desired result)...

 

[mivey]

Yes, but in the case of a 20A OCPD on the secondary, with two 120V coils wired in parallel, and rated for 3kVA cumulatively, one failing could result in the other pulling up to 2400VA on a 1500VA rated coil without tripping the secondary OCPD. With a single coil rated for 3kVA, it could never surpass the rated kVA without tripping the secondary OCPD.

Unless I misunderstand transformer kVA ratings, which is entirely possible, and both secondary coils are rated for 3kVA in a single transformer rated for 3kVA.

So what? That will not overload the supply conductors. You would double the voltage drop and overheat the transformer, both of which could be monitored for a separate transformer protection scheme.

The idea of the primary/secondary protection is that a secondary fault will be seen in a direct per-unit value on the primary. Other configurations might see a reduced percentage at primary and thus a secondary fault might not trip the primary and you have then lost your secondary conductor protection blanket.

Protecting the transformer from overloads is a separate issue. Many primary protection schemes will not prevent long term overloads even in normal operation. If needed, that is done with things like thermal overloads, additional relaying, etc.

You are mixing different concerns that are addressed different ways.

 

[kwired]

That's exactly what I'm saying.

This section of code is telling me that, if I want to NOT have secondary conductor protection, two conditions must be met regarding (1) the transformer secondary conductors and (2) the transformer secondary itself.

The moment I wire this transformer for a single phase 3-wire output, secondary conductor protection is required.

The part that's being debated is the differences between sentence #1 and sentence #2 of 240.4(F). I'm saying sentence number #1 gives conditions for the secondary conductors and sentence #2 gives conditions for the actual secondary of the transformer... both of which are conditions for either requiring secondary conductor protection or allowing secondary conductors to go without.

You must have secondary conductor protection, there are times where primary protection can accomplish that and no additional secondary protection necessary.

 

[jim dungar]

When you say "each winding is at the rated for the maximum kVA at the maximum voltage," do you mean each INDIVIDUAL COIL (1 out of 4)...

OR "each winding" as in "either the primary or secondary side(s)" because there is no way to wire this particular xfmr to manufacturer's specifications without utilizing both coils on either side (i.e. wired in series or parallel to produce the desired result)...

There are four windings on the transformer in your OP, 2 on the primary and 2 on the secondary. Their connection points are H1-H3, H2-H4, X1-X3, and X2-X4.
I do not know how many coils make up each winding.

 

[Jerramundi]

There are four windings on the transformer in your OP, 2 on the primary and 2 on the secondary. Their connection points are H1-H3, H2-H4, X1-X3, and X2-X4.
I do not know how many coils make up each winding.

Okay, four "windings" on the transformer in the OP. Are you saying EACH ONE of those 4 is rated for 3kVA? I'm operating under the assumption that Primary #1 + Primary #2 = 3kVA... and Secondary #1 + Secondary #2 = 3kVA. Just want to be sure I correctly understand kVA ratings.

 

[Jerramundi]

So what? That will not overload the supply conductors. You would double the voltage drop and overheat the transformer, both of which could be monitored for a separate transformer protection scheme.

The idea of the primary/secondary protection is that a secondary fault will be seen in a direct per-unit value on the primary. Other configurations might see a reduced percentage at primary and thus a secondary fault might not trip the primary and you have then lost your secondary conductor protection blanket.

Protecting the transformer from overloads is a separate issue. Many primary protection schemes will not prevent long term overloads even in normal operation. If needed, that is done with things like thermal overloads, additional relaying, etc.

You are mixing different concerns that are addressed different ways.

With the particular issue I've described here (i.e. one coil on the secondary side failing), I'm not concerned about the secondary conductors, but the transformer secondary itself. Yes, across various posts I've addressed both (1) the secondary conductors and (2) the transformer secondary itself, but I'm aware of where the line is between them.

I don't think the failure I've described would "double the voltage," but rather double the amperage across the remaining coil. Per my understanding of kVA ratings (i.e. Secondary Winding #1 + Secondary Winding #2 = 3kVA on a 3kVA rated Xfmr), the remaining secondary winding would be pulling a higher kVA than it is rated for before tripping the Secondary OCPD.

All of this was to simply say that going without secondary protection would likely be better for a 100% true, as manufactured, single winding, single voltage, two-wire secondary. Which, IMHO, adds support to the argument that a multiple coil secondary w/ the potential for multiple voltage outputs, even when wired for 120V, may not be an acceptable form of a "2-wire (single voltage) secondary" with respect to not installing secondary protection.

 

[Jerramundi]

You must have secondary conductor protection, there are times where primary protection can accomplish that and no additional secondary protection necessary.

Perhaps a poor choice of wording, but I think you know that I meant "primary protection only" when I said "if I want to NOT have secondary conductor protection." Or at least I feel like you should in the context of this whole thread. But yes, begrudgingly I admit that is a valid clarification. *grumble* *grumble*

 

[winnie]

All of this was to simply say that going without secondary protection would likely be better for a 100% true, as manufactured, single winding, single voltage, two-wire secondary. Which, IMHO, adds support to the argument that a multiple coil secondary w/ the potential for multiple voltage outputs, even when wired for 120V, may not be an acceptable form of a "2-wire (single voltage) secondary" with respect to not installing secondary protection.

If you have a 3KVA transformer constructed with 2 secondary coils which are 'field wired' in parallel, the secondary protection would be for the _parallel_ coils, not for the individual coils.

This is what I meant by a clear distinction that does not make a practical difference. Yes, if you have two coils in parallel versus a single coil, you have a potential failure mode where one of the parallel coils gets disconnected and the other coil gets overloaded. However secondary protection _if required_ would not protect from this failure mode. In cases where secondary protection is required, it is for the _entire_ secondary, not portions of the secondary.

Now if you had a 20A secondary consisting of two parallel 10A coils, with separate 10A OCPD in each coil that would be a different story, but NEC doesn't require this.

-Jon

 

[jim dungar]

Okay, four "windings" on the transformer in the OP. Are you saying EACH ONE of those 4 is rated for 3kVA? I'm operating under the assumption that Primary #1 + Primary #2 = 3kVA... and Secondary #1 + Secondary #2 = 3kVA. Just want to be sure I correctly understand kVA ratings.

What I said was; each winding is rated for a specific amount of current.
Yes, for a two winding connection you could say it effectively means each one is roughly 1/2 of the total kVA for the connection.

 

[LarryFine]

There are four windings on the transformer in your OP, 2 on the primary and 2 on the secondary. Their connection points are H1-H3, H2-H4, X1-X3, and X2-X4.
I do not know how many coils make up each winding.

My position is how on Earth can two secondary windings qualify as a "single voltage, two wire secondary" ?

 

[LarryFine]

I'm operating under the assumption that Primary #1 + Primary #2 = 3kVA... and Secondary #1 + Secondary #2 = 3kVA. Just want to be sure I correctly understand kVA ratings.

This is correct. The power rating is for the entire unit as a whole.

 

[david luchini]

My position is how on Earth can two secondary windings qualify as a "single voltage, two wire secondary" ?

 

[jim dungar]

My position is how on Earth can two secondary windings qualify as a "single voltage, two wire secondary" ?

The OP nameplate shows 3 different winding configurations. Two of them provide a single voltage with two wire output. How do they not meet the requirement? The internal connections of a transformer are not subject to the NEC, they are part of the listing requirements. The NEC is concerned with the nameplate.

240.4(F) references 450.3 (its definition of transformer can be found in 450.2). 450.2 refers to a single nameplate as part of the description. Nowhere does the NEC mention the number of coils that make up individual windings, nor the number of windings used to make up a transformer.

Do you consider a single winding with a center tap, which is not wired, as meeting 240.4(F) and 240.21(C)1?